Ribbon microphone



March 25, 1969 c. P. FISHER 3,435,143

RIBBON MI CROPHONE Filed Aug. 2, 1965 Sheet of 2 i INVENTOR CHARLES P.FISHER ATTORNEYS March 25, 1969 c. P. FISHER RIBBON MICROPHONE SheetFiled Aug.

EMITTER FOLLOWER AMPUFIER INVENTOR CHARLES P HS BY, I

FIG?

ATTORNEYS CYCLES PER SECOND IOOO United States Patent Ofitice 3,435,143Patented Mar. 25, 1969 3,435,143 RIBBON MICROPHONE Charles P. Fisher,Framingham, Mass. (473 Washington St, Wellesley, Mass. 02181) Filed Aug.2, 1965, Ser. No. 476,290 Int. Cl. H04m 1/00; H04r 9/00, 9/08 US. Cl.179-1 13 Claims ABSTRACT OF THE DISCLOSURE In a ribbon microphone, thewidth of the ribbon, the cross sectional area of the poles, and theseparation between the poles is varied along their common length therebymaintaining saturation of the poles and a uniform magnetic field in theribbon gap. A feedback circuit is used to reduce the bass sensitivity ofthe transmission.

The present invention relates in general to electroacousticaltransduction and more particularly concerns an improved ribbonmicrophone characterized by good frequency response and directivitywhile providing a relatively noise-free output signal capable of beingtransmitted over relatively long distances without picking up undesirednoise components. Yet, the assembly is compact and uses relatively fewcomponents.

A ribbon microphone typically comprises a very low mass conductorsupported in a magnetic field. This conductor, or ribbon, responds tothe pressure differential between its two faces to provide an electricalsignal representative of the velocity component of an incident soundwave. The magnetic field is typically furnished by one or more permanentmagnets and delivered to the region of the ribbon by soft iron poles.These poles act as baflles establishing the pressure differential towhich the ribbon responds. At high frequencies, sound energy isdiffracted around the poles to reach the back of the ribbon in phaseopposition to the sound energy incident upon the front of the ribbon andproduces cancellation efiects. Ribbon microphones are thus limited inhigh frequency response by the Width dimension of the poles. Since theelectrical signal for a given sound pressure level is proportional tothe product of magnetic field density, ribbon length and ribbonvelocity, it is desirable to have a strong magnetic field surroundingthe vibrating ribbon. But dense magnetic fields call for relativelymassive poles, a requirement apparently in conflict with the requirementfor good high frequency response; namely, that the poles be slender.Increasing the length of the ribbon and the poles does increase signal,but when the length approaches a wavelength at sound frequencies,off-axis response of the microphone exhibits a sharp decrease fromcancellation effects due to opposing signal phases near the ends of theribbon.

The ribbon itself is a mass responsive to the moving viscous air medium.Its own velocity will therefore be less than that of the medium. If theribbon mass is decreased, for example by making it thinner, its velocitywill be a higher percentage of that of the sound wave in the surroundingmedium. At the limit, when the mass is zero, this percentage would beone hundred percent. Practical considerations limit the thinness of theribbon material. Using the usual material, aluminum, it has been foundthat if the ribbon thickness is reduced below about 0.1 mil, theresistance increases faster than the velocity squared so that the signalpower output decreases.

An especially severe limitation resides in the saturation density of thesoft iron used in the poles. This density could be made available in thegap embracing the ribbon,

0 but the required thickness of the poles would be so great 7 as to seta relatively low top limit on frequency response.

Accordingly, it is an important object of this invention to overcome thedifficulties enumerated above and provide an improved ribbon microphonecharacterized by a wide frequency response, satisfactory sensitivity,the provision of a relatively low noise output signal relatively easy totransmit over relatively long distances with relatively littleadditional increase in noise, all embodied in a relatively compactphysical structure.

According to the invention, a vibratable ribbon means is situated in amagnetic field provided by opposed poles. This apparatus is supported ina housing about the size of a conventional microphone which alsoincludes amplifying means responsive to the electrical signal providedby the ribbon means to provide an output signal relatively free fromnoise at a low impedance level suitable for transmission over relativelylong distances. Preferably, switchable feedback means are providedintercoupling the amplifying means output and the ribbon means so as toroll olf the effective bass response of the system and reduce thesensitivity of the system effectively to bass signal sources close tothe ribbon means.

In a preferred form of the invention the width of the ribbon, the crosssectional area of the poles and the separation between the poles isvaried along their common length so as to maintain saturation of thepoles and approximately uniform magnetic field density in the gapimmediately adjacent to the ribbon means. The resulting poleconfiguration comprises conical tapered poles which converge at theirsmall ends and a ribbon means tapered similarly to a norrow width at oneend and considerable width at the other so that its overall resistanceis relatively low.

According to theoretical consideration believed to be applicable to theobjects of this invention, profiles of the opposed pole faces arepreferably of parabolic outline for their cross section at all points tobe precisely in proportion to the flux requirement. In practice theyhave been formed as truncated cones with extra material being left onthe small ends with the discrepancy between desired and actual fluxdensity being minor from the practical standpoint. An important usefuleffect of this type of construction is that at the small end of thepoles, the baflle effect is negligible so that the off-axis responseabove 15 kilocycles is much better than that predicted for pole faces ofuniform and larger cross section. Practically, good response anddirectivity are still provided at 20 kilocycles.

Numerous other features, objects and advantages of the invention willbecome apparent from the following specification when read in connectionwith the accompanying drawing in which:

FIG. 1 is a perspective view of a typical microphone system according tothe invention;

FIG. 2 is an enlarged perspective view of the ribbon and magnetic fieldforce structure;

FIG. 3 is an exploded view of the means for fastening the narrow end ofthe ribbon;

FIG. 4 is a side view of the structure shown in FIG. 2;

FIG. 5 is a top view taken through section 55 of FIG. 4;

FIG. 6 is a combined block, pictorial, schematic-circuit diagram of apreferred embodiment according to the invention; and

FIG. 7 shows the response change due to the switchable bass roll-0Emeans embodiment shown in FIG. 6.

With reference to the drawing and more particularly FIG. 1 thereof,there is shown a microphone system according to the invention suspendedby the spring 11 and U-shaped bracket 12 so that sound incident upon theacoustically transparent cage 13 surrounding the ribbon assembly istransduced into an electrical signal relatively free of noise andpresented at a convenient impedance level for transmission overrelatively long distances without picking up appreciable undesired noiseo-ver output cable 14. Batteries 15 and 16, which provide power for theinternally contained amplifying means, are shown mounted on the top ofhousing 17 to permit convenient battery replacement.

Referring to FIG. 2, there is shown a perspective view of a preferredembodiment of the ribbon means and source of magnetic field. A U-shapedpermanent magnet comprising the two sections 21 and 22 comunicates withthe left pole piece 23 and the right pole piece 24 which terminate in aleft bafile 25 and a right baffle 26, respectively. Baffles 25 and 26define the air gap which decreases in width as a function of increasingheight in which the correspondingly shaped ribbon 27 vibrates. The longedges 31 and 32 of ribbon 27, are closely adjacent to but spaced frombafiles 25 and 2-6, respectively, so that they embrace essentially themaximum amount of flux in the gap while being free to vibrate withoutrubbing against baffles 25 and 26. The wide bottom edge 33 is clamped inlower support 34 which in turn is supported with its top surface againstpole pieces 23 and 24. The upper narrow edge of ribbon 27 is similarlyclamped in horizontal member 35 by means including a brick 38 secured tomember 35 by means including the screws 39 residing in tapped holes 40,shown more clearly in FIG. 3.

Horizontal supporting member 35 is sandwiched between a pair of leftinsulating washers 36 and right insulating washers 37 and a pair of leftnuts 41 and a pair of right nuts 42 which mate with threaded left andright studs 43 and 44, extensions of left vertical support 45 and rightvertical support 46, respectively. By adjusting the nuts 41 and 42, thetension in ribbon 27 may be adjusted. Conductors '46 are arranged as arectangular loop to prevent sensitivity to stray fields because theribbon bisects the loop.

Referring to FIGS. 4 and 5, there is shown a side view of the structureshown in perspective in FIG. 2 and a top view through section 5 of FIG.4, respectively. In these figures the reference numerals of FIG. 2identify corresponding elements.

A pair of conductors 46 are soldered to clamping means 35 and 38 andprovide a very low resistance path from the upper edge of ribbon 27 tothe transformer input.

Referring to FIG. 6, there is shown a combined pictorial blockschematic-circuit diagram of a preferred system according to theinvention having switchable bass roll-off to compensate for the peak inbass response usually exhibited by a ribbon microphone when in the nearfield of a bass signal source. A transformer 47 has its primary 51connected between right pole face 24 and conducting leads 46 totransformer-couple the low level signal developed across ribbon 27 intothe input of transistor amplifier 51. The amplified signal fromamplifier 51 is applied to emitter follower 52 to provide the amplifiedsignal on output line 53 at a convenient impedance level so that theoutput signal may be carried by output cable 14 (FIG. 1) over relativelylong distances without undesirable pickup. Batteries and 16 supply powerto amplifiers 51 and 52 when switched on by double pole-single throwswitch A feedback network comprising resistors 54 and 55 and capacitor56 with the parameter values indicated functions with switch 57 closedto provide the bass rolloif shown in FIG. 7 when the resistance ofribbon 27 is approximately 0.5 ohm.

In a specific embodiment of the invention the ribbon was made ofaluminum .08 mil thick, 1% inches long and tapering from .200 inch atthe bottom to .07 inch at the top. Bafiies and 26 were of cobalt-ironwith circular cross section at the bottom .33 inch in diameter, 1 incheslong and .13 inch in diameter at the top. Pole pieces 23 and 24 weremade of mild steel material. Magnets 21 and 22 were made of Almcomaterial and weighed a total 4 of 36 ounces. Amplifier 51 was a P65 typePhilbrick amplifier with a gain of 20 and emitter follower 52 was 21 P66 type Philbrick emitter follower amplifier. Battery 15 was +15 voltsand battery 16 was 15 volts. Transformer 47 was an audio transformerwith a turn ratio of :1.

With a sound pressure of one microbar, the voltage at ribbon 27 was 7microvolts; at the secondary of transformer 47, 1.25 millivolts; and atoutput 53, 25 millivolts, making the total voltage gain essentially4,000.

It is evident that those skilled in the art may now make numerousmodifications of and departures from the specific embodiment describedherein and the specific dimensions and parameters set forth withoutdeparting from the inventive concepts. Consequently, the invention is tobe construed as limited solely by the spirit and scope of the appendedclaims.

What is claimed is:

1. Electroacoustical transduction apparatus comprising vibratable ribbonmeans,

means including a pair of opposed pole means for providing a magneticfield,

means for supporting said vibratable ribbon means between said opposedpole means and in said magnetic field, the width of said vibratableribbon means, the cross sectional area of each of said opposed polemeans and the separation between said opposed pole means varying along alength common to said vibratable ribbon means and said opposed polemeans so as to both maintain saturation of said pole means and establishapproximately uniform magnetic field density in the gap between saidopposed pole means immediately adjacent to said vibratable ribbon means,

means responsive to vibration of said vibratable ribbon means forproviding a low level electrical signal, and

amplifying means responsive to said low level signal for providing anoutput signal relatively free from noise at a low impedance levelsuitable for transmission over relatively long distances.

2. Electroacoustical transduction apparatus in accordance with claim 1and further comprising:

feedback means intercoupling the amplifying means output and saidvibratable ribbon means for rolling off the effective bass response ofsaid apparatus and reducing the sensitivity of said apparatuseffectively to bass signal sources close to said vibratable ribbonmeans.

3. Electroacoustical transduction apparatus in accordance with claim 1wherein said means responsive to said low level signal includes aconductor connected to said vibratable rib bon means and forming agenerally rectangular loop that is substantially bisected by saidvibratable ribbon means with the long sides of said loop substantiallyparallel to the length of said vibratable ribbon means.

4. Electroacoustical transduction apparatus in accordance with claim 1and further comprising:

transformer means for coupling said low level electrical signal to saidamplifying means.

5. Electroacoustical transduction apparatus in accordance with claim 1wherein said opposed pole means comprise conical tapered pole elementswhich converge toward one another at their small ends so that theseparation between said small ends is less than that between the largeends of the latter elements and said vibratable ribbon means is taperedsimilarly to a narrow width at the end near said small ends and largerwidth at the other while being characterized by relatively lowresistance between its narrow width end and its larger width end.

6. Electroacoustical transduction apparatus in accordance with claim 5and further comprising:

feedback means intercoupling the amplifying means output and saidvibratable ribbon means for rolling off the eifective bass response ofsaid apparatus and reducing the sensitivity of said apparatuseffectively to bass signal sources close to said vibratable ribbonmeans.

7. Electroacoustical transduction apparatus in accordance with claim 5wherein said means responsive to said low level signal includes aconductor connected to said vibratable ribbon means and forming agenerally rectangular loop that is substantially bisected by saidvibratable ribbon means with the long sides of said loop substantiallyparallel to the length of said vibratable ribbon means.

8. Electroacoustical transduction apparatus compris- 111g:

vibratable ribbon means,

means including a pair of opposed pole means for providing a magneticfield,

means for supporting said vibratable ribbon means between said opposedpole means and in said magnetic field,

the width of said vibratable ribbon means, the cross sectional area ofeach of said opposed pole means and the separation between said opposedpole means varying along a length common to said vibratable ribbon meansand said opposed pole means so as to both maintain saturation of saidpole means and approximately uniform magnetic field density in the gapbetween said opposed pole means immediately adjacent to said vibratableribbon means.

9. Electroacoustical transduction apparatus in accordance with claim 8wherein said means responsive to said low level signal includes aconductor connected to said vibratable ribbon means and forming a loopthat is substantially bisected by said vibratable ribbon means.

10. Electroacoustical transduction apparatus in accordance with claim 8wherein said means responsive to said low level signal includes aconductor connected to said vibratable ribbon means and forming agenerally rectangular loop that is substantially bisected by saidvibratable ribbon means with the long sides of said loop substantiallyparallel to the length of said vibratable ribbon means.

11. :Electroacoustical transduction apparatus in accordance with claim'8 wherein said opposed pole means comprise conical tapered poleelements which converge toward one another at their small ends so thatthe separation between said small ends is less than that between thelarge ends of the latter elements and said vibratable ribbon means istapered similarly to a narrow width at the end near said small ends andlarger width at the other while being characterized by relatively lowresistance between its narrow width end and its larger width end.

12. Electroacoustical transduction apparatus in accordance with claim 11wherein said means responsive to said low level signal includes aconductor connected to said vibratable ribbon means and forming a loopthat is substantially bisected by said vibratable ribbon means.

13. Electroacoustical transduction apparatus in accordance with claim 11wherein said means responsive to said low level signal includes aconductor connected to said vibratable ribbon means and forming agenerally rectangular loop that is substantially bisected by saidvibratable ribbon means with the long sides of said loop substantiallyparallel to the length of said vibratable ribbon means.

References Cited UNITED STATES PATENTS 2,093,664 9/ 1937 Swickard.2,493,819 1/1950 Harry. 2,495,809 1/ 1950 Graham. 3,130,374 4/1964 Bereset a1. 179-1.2 3 ,293,3 64 12/ 196 6- Richter 179-1 .2

WILLIAM C. COOPER, Primary Examiner.

R. P. TAYLOR, Assistant Examiner.

US. Cl. X.R. 179-138, 119

